Technical Field of the Invention
Aspects of the present disclosure relate in general to optical radiation sources, and in particular to silicon-based thermal emitters.
Description of Related Art
Conventional thermal light sources utilize thermal emitters that generate photons by heating emitting material. Such thermal emitters are typically broadband radiators whose output spectral radiance is a function of the wavelength, the absolute temperature of the heated material and the emissivity of the element, all of which are governed by Planck's Law. The ideal source of continuous near- to mid-infrared (NIR) radiation is a high-temperature blackbody. The spectral brightness Uλ (e.g., the power flow per unit area per wavelength per steradian at wavelength λ) from a blackbody source at temperature T is given by Planck's equation (in W/mm2.μm.sr):
                                                        U              λ                        ⁡                          (                              λ                ,                T                            )                                =                                    C              1                                                      λ                5                            ⁡                              [                                                      exp                    ⁡                                          (                                                                        C                          2                                                ⁢                                                  /                                                ⁢                        λ                        ⁢                                                                                                  ⁢                        T                                            )                                                        -                  1                                ]                                                    ,                            (                  Equation          ⁢                                          ⁢          1                )            where C1=11.9 (W. μm4/mm2.sr) in a vacuum, C2=14390 μm.K, T is the absolute temperature of the radiating body, and λ is the wavelength. Thus, according to Planck's equation, for any particular wavelength range, the emitted radiance increases with temperature.
The blackbody is the perfect emitter and absorber of radiation, and it also radiates uniformly in all directions per unit area normal to the direction of emission. However, for practical non-ideal sources, the radiation properties are less than that of a blackbody. Therefore, the emissivity of a surface (0≦ε≦1) may be represented as the ratio of the radiation emitted by the surface to the radiation emitted by a blackbody at the same temperature, and the emissivity may vary with temperature, wavelength and direction of the emitted radiation.
When a radiation flux is incident on a surface, the incident radiation flux is commonly referred to as irradiation, and at the surface, part of the irradiation is absorbed, part is reflected and the remaining is transmitted. The absorptivity α is the fraction of irradiation absorbed by the surface. Kirchhoff's law states that the total emissivity of a surface at temperature T is equal to its total absorptivity of radiation from a blackbody at the same temperature.
Most of the thermal radiation products currently available are based on filament incandescent sources that approximate a blackbody with a special glass bulb for use in the low NIR or NIR spectral range. Their operating temperatures can reach up to 2000-3000 K, due to the glass and the inert gas inside the bulb. The spectral radiation of these filaments can extend up to 4 μm, as the glass absorbs wavelengths beyond this limit. However, the package geometry of such glass-bulb based filament incandescent sources may be problematic for some applications.
Filament incandescent sources without a glass bulb are also available for some applications. Operating in an open environment, their temperature is usually limited to 600-900 K, leading to low emitted power for a given filament surface area. To increase the temperature, the filament can be hermitically sealed with a transparent window in the desired wavelength range. For example, a Calcium Fluoride window may be used for wavelengths up to 8 μm. However, such incandescent sources are not compatible with device miniaturization and batch fabrication in photonics applications.
Microelectromechanical system (MEMS) silicon technology is a rich platform for the integration of electrical, mechanical and optical systems on a chip. Single-crystal silicon material has excellent purity and well-established optical properties, which include good surface quality and transparency over a wide spectral range in the infrared range. Furthermore, silicon has excellent thermal, mechanical and thermo-elastic properties. Moreover, there is great flexibility in the design and manufacture of silicon devices, taking advantage of well-established microfabrication technologies.
Therefore, thermal sources formed from planar silicon structures have been developed for the mid-infrared. These sources usually consist of thin film structures fabricated onto a silicon substrate. A resistive heating element may be integrated on the silicon substrate to heat the emission area. These MEMS thermal sources have low electrical power consumption due to their low thermal mass as a result of the substrate being made very thin.
MEMS sources may further be optimized to provide good emissivity across a large wavelength range by increasing the surface roughness. For example, black platinum or black silver may be deposited on the surface of a silicon emitter to increase the surface roughness. As another example, photonic crystal deep cavities of silicon may be fabricated on the silicon substrate to increase the surface roughness. Such a periodic structure may be obtained by photolithography followed by electrochemical etching and oxidation to form stable cavities of porous silicon. However, the spectral response of the emissivity can significantly vary with the pore size and the periodicity of the cavities can result in sharp dip lines in the emissivity. In addition, the emission covers a broad wavelength range that may not be usable in all applications, and thus, may represent a deficiency. Therefore, what is needed is a miniaturized tunable thermal emitter that maximizes the surface area and emissivity over a wide emission band.